This invention relates to an apparatus for reducing noise mixed in a video signal, which is used in a television receiver or a video cassette recorder and so forth, and more paticularly it relates to a circuit for controlling a characteristic of noise reduction in accordance with a degree of motion of an image detected on the video signal.
FIG. 1A shows an arrangement of a conventional noise reduction apparatus. The apparatus includes two types of noise reduction processing circuits 2 and 5. The frame type processing circuit 2 functions to detect a correlation between a frame of video signal and the next frame by comparing them and reducing the noncorrelating parts as being the noise mixed in the video signal. For example, a frame type processing circuit can comprise an average circuit which produces an average signal having an intermediate level of two successive frames of the video signal, so that the noncorrelating parts of the frames are reduced. A memory 3 is provided for storing the previous frame of the video signal used for comparison. A line type noise processing circuit 5 detects a correlation between a horizontal line of the video signal and the next line and reduces noncorrelating parts. This line type circuit 5 can also comprise another average circuit.
A video signal at an input terminal 1 is supplied to the noise reduction processing circuits 2 and 5, and two output signals SF and SL are supplied to a mixing circuit 6. The mixing circuit 6 mixes the noise reduced signals SF and SL at a mixture rate which is determined by receiving a motion detecton signal MDS produced by a motion detector 4, and supplies a mixed signal MS to an output terminal 7. The motion detector 4 detects to what degree each noncorrelating part between two successive frames of the video signal has a noncorrelation. The detector 4 compares the two frames by the picture element in order to carry out the detection, and it uses the memory 3 in the same manner as the frame type processing circuit 2. The motion detection signal MDS is selected in accordance with the result of the detection and corresponds to the mixture rate of the mixing circuit 6.
For example, the mixing circuit 6 mixes the signal SF with the signal SL according to the formula: EQU SF.multidot.k+SL.multidot.(1-k), 0.ltoreq.k.ltoreq.1.
The variable k depends on the motion detection signal MDS. If the signal MDS represents, as the result of the detection, that there is a large difference at a noncorrelating part, the variable k is set to a numerical value close or equal to "0". In this case, the noncorrelating part between two successive frames seems to be caused not by a noise but by a motion of the picture. Therefore, the frame type noise processing circuit 2 is limited from its performance because the noncorrelating part detected thereby is not a noise, instead the line type noise processing circuit 5 is more activated. On the other hand, if the motion detection signal MDS represents that the difference at the noncorrelating part is less than a predetermined level or two successive frames have a correlation, the variable k is set to numerical value being close or equal to "1". In such case, the image of the video signal seems like a still picture. Then the performance of the line type noise processing circuit 5 is limited because it is liable to make the picture less clear, instead the frame type circuit 2 is made the most of the ability.
However, the above mentioned conventional noise reduction apparatus can not reduce a noise having a correlation between two successive frames. For example, the noise somtimes occures at a fixed position on a screen. As shown in FIG. 1B, when the successive frames A and B have the same noises at the same positions on the same lines n and n+1, the frame type noise processing circuit 2 can not reduce the noises because there is no noncorrelating part between the two frames. Meanwhile the line type noise processing circuit 5 detects and reduces the noncorrelating part between the two lines. However, the motion detector 4 judges that the frames A and B have a correlation, so it transmits the motion detection signal MDS in order to make the mixing circuit 6 select the output signal SF from the frame type circuit 2 rather than the output signal SL from the line type circuit 5. Accordingly, the output signal MS from the mixing circuit 6, which corresponds to a frame C shown in FIG. 1B, still includes the noise.
The conventional noise reduction apparatus is subject to another problem. FIG. 1C shows an example of each signal waveform. When a horizontal line Sa of the video signal is different from the same line Sb of the next frame as shown in FIG. 1C, the motion detection signal MDS changes a point on the horizontal line at where a difference of amplitude between the two signals Sa and Sb reaches a predetermined level. In this example, the motion detection signal MDS is produced to have either a low level representing that variable k of the above formula is set equal to "1", or a high level representing that the variable k is set equal to "0". Therefore, the signal SF from the frame type circuit 2 passes through the mixing circuit 6 when the signal MDS is low, and oppositely the signal SL from the line type circuit 5 passes through the mixing circuit 6 when the signal MDS is high. In the case shown in FIG. 1C, the output signal MS of the mixing circuit 6 changes at a point on the horizontal line. Then, the signal MS has a waveform distortion at the change over point because of the difference of amplitude between the signals SF and SL. The waveform distortion appears on a screen.
Another type of conventional noise reduction apparatus uses a field type noise reduction processing circuit instead of the frame type circuit 2, which reduces noncorrelating parts between two successive fields of the video signal in the same manner as the frame type circuit 2, and it uses a motion detector which detects a difference between the two fields.